In any coating process of an article of manufacture such as an automotive body, there is an optimum specification for the resulting film build, i.e., thickness of the resulting coating layer involving acceptable performance, appearance and materials cost. The ability to measure this film build on-line in a production environment would be beneficial to the manufacturer.
Often, any method for measuring the film build of a coating layer must require that no contact with the film occur either to avoid degrading the effectiveness or marring the appearance of the film.. This is especially true for coatings while they are wet.
With manufacturing film build data, the bulk materials costs can be controlled by applying the minimum amount of material to reach an acceptable film build. Other savings can also be realized, for example measuring and improving the transfer efficiency of the coating process and correlating film build to the quality of the appearance of the final coated surface. An example process and production environment that would benefit from the ability to measure film build on-line is the painting of automobile bodies.
In automated painting operations, a prime concern is the reduction of environmental impacts due to the evaporation of solvents. Means of reducing the amount of solvent released into the atmosphere include electrostatic application of the paint and the use of waterborne paints. Electrostatic application increases the quantity of paint delivered to the painted object, and thus reduces the total quantity of paint required due to the decrease in overspray. The use of waterborne paints dramatically reduces the quantity of solvent utilized in the paint because water is used as a vehicle for paint delivery rather than solvent. Environmental concerns may dictate the exclusive use of waterborne paints in the future.
In order to further reduce waste, thus reducing solvent emissions, and to improve the quality of the finished painted article, it may be necessary to monitor or sense various paint physical parameters such as thickness with precision to effect control.
Waterborne paints are electrically conductive and, therefore, must be isolated from the environment such that an electrostatic charge may be imparted to the flow of paint. This isolation must be at least 100 kilovolts. Further, the painting environment is a hazardous environment due to the few remaining solvents in the paint. Therefore, any device which meters or measures the physical parameters of paint must provide electrostatic isolation and limit energies within the painting environment to less than that required for ignition.
The painting process for automobiles involves applying several coatings of different materials to an underlying metal or plastic substrate 10. As illustrated in FIG. 1, these coatings may include various anticorrosion layers such as a phosphate layer 12, an E-coat layer 14, primer layer(s) 16, colored paint layers 18 (referred to as base coats), and a transparent protective and appearance improving material(s) called a clearcoat 20. The ability to measure both total film build, i.e., the total thickness of all layers and/or the thickness of each individual layer, in both the wet or dry states would be useful.
One non-contact method for measuring solid film thickness and/or other physical properties of the film is provided by ultrasound generation in the film and subsequent ultrasound detection. However, this method typically locally damages or destroys the film.
For example, U.S. Pat. No. 4,659,224 discloses optical interferometric reception of ultrasound energy using a confocal Fabry-Perot interferometer and the detection of the Doppler shift of the laser line frequency as the method to detect the ultrasound.
U.S. Pat. No. 5,137,361 discloses optical detection of a surface motion of an object using a stabilized interferometric cavity. The interferometer cavity is stabilized by controlling the position of the rear cavity mirror with a piezoelectric pusher.
U.S. Pat. No. 5,402,235 discloses imaging of ultrasonic-surface motion by optical multiplexing. Ultrasound is detected using an array of detectors and a "demodulator". The demodulator is typically a photorefractive crystal within which a hologram of the laser beam, both directly from the laser and reflected off the sample's surface, are simultaneously written. The interference between sample laser beam and the beam reflected off the sample surface takes place between the two holographic images generated within the crystal.
U.S. Pat. No. 5,590,090 discloses an ultrasonic detector using vertical cavity surface emitting lasers. The method requires contact between the sample and the equipment.
U.S. Pat. No. 5,035,144 discloses frequency broadband measurement of the characteristics of acoustic waves. Propagating acoustic waves are detected by two different broadband receivers at first and second locations separated by a distance L. The data analysis for this method involves detailed comparisons between group and phase velocities of the data using different amplifiers and narrow band filtering of the signal.
U.S. Pat. No. 5,604,592 discloses a laser ultrasonics-based material analysis system and method using matched filter processing. The system uses a diode laser for detection. Generation and detection of ultrasound is done at different points. The system relies on Time of Flight (TOF) detection which requires generation and detection at separate points since the basis of the measurement is the time it takes for the ultrasonic energy to travel between the two points. The waveshape of the time varying ultrasonic signal is acquired with a matched filter, processed and basically compared to a library of similar signals.
U.S. Pat. No. 5,615,675 discloses a method and system for 3-D acoustic microscopy using short pulse excitation and a 3-D acoustic microscope for use therein.
U.S. Pat. No. 5,305,239 discloses ultrasonic non-destructive evaluation of thin specimens. The method involves data analysis for thin specimens, where "thin" is defined as less than or equal to the wavelength of the inspecting acoustic wave. Analysis is demonstrated with a Fast Fourier Transform (FFT). An important aspect of an FFT is that it can only produce discrete frequency results determined by the number of points taken and data rate used.